Thermocouple measuring tubes are known in the art to measure pressure in a partial vacuum. A change in pressure in the measuring tube changes the molecular collision rate and therefore the thermal conduction of the gas or gas mixture surrounding the thermocouple. Such measuring tubes are typically operated using one of two alternative operating methods.
In a first operating method, an alternating voltage is continuously applied to the thermocouple, thereby heating the thermocouple. The resulting temperature shift depends on the pressure of the surrounding gas or gas mixture and causes a change in the thermocouple's DC output inversely with pressure changes. Unfortunately this operating method can only be used within a relatively small pressure measurement range. Examples utilizing this operating method are DV-4 and DV-6 thermocouple vacuum gauge tubes manufactured by Teledyne Hastings.
In a second operating method, the thermocouple's temperature is electronically controlled to maintain a predetermined temperature by continuously adjusting the electrical power applied to the thermocouple. The amount of power applied to the thermocouple is evaluated and used as a measure for the pressure of the partial vacuum. This operating method extends the measurement range of pressures in a vacuum, but not by much.
U.S. Pat. No. 4,579,002, hereby incorporated by reference thereto in its entirety, describes an operating method, in which a pulsed heating current is supplied to the thermocouple. In the “off” periods of the heating pulse the generated thermoelectric voltage (EMF) is measured using an amplifier. A comparator compares the measured, amplified thermoelectric voltage to a set-point value. If the amplified EMF deviates from the set-point value, the length of the heating pulse is adjusted to move the amplified EMF towards the set-point value. This maintains a constant temperature at the thermocouple while operated.
A similar method is described in U.S. Pat. No. 5,351,551, in which the constant temperature at the thermocouple is maintained by controlling the current of the heating pulse. U.S. Pat. No. 5,351,551 is hereby incorporated by reference thereto in its entirety.
U.S. Pat. No. 6,727,709, hereby incorporated by reference thereto in its entirety, describes a thermal conduction vacuum gauge using a Peltier tip. The Peltier tip is part of a measuring bridge in a vacuum chamber. The measuring bridge may be operated at constant power or at constant temperature. A voltage signal obtained from the measuring bridge is a measure of the pressure.
DD 249 534 A1 describes a method and a system for measuring the pressure of gases. The partial vacuum is measured by use of a current-carrying electrical measuring resistor whose resistance changes with temperature. The measuring resistor is heated by high-voltage pulses. During the “off” periods of the heating pulse, low voltage is applied to the measuring resistor in order to determine its resistance upon cooling. The cooling time following a heating pulse is used as a measure of the pressure. A disadvantage is that the low voltage applied during cooling influences the cooling time.
EP 1 409 963 B1 by the same applicant, which is hereby incorporated by reference thereto in its entirety, describes sensors and methods for detecting measurement variables and physical parameters. Sensors, for example Pirani measuring elements, whose electrical resistance changes as the result of current flow and the associated temperature increase, are supplied with electrical pulses. The amplitude of these pulses is changed over time according to a mathematical function.
During a measuring cycle the voltage of the pulses may, for example, be increased linearly over time. Correspondingly, the power to the sensor measuring element increases quadratically over time. For a Pirani gauge operated in this manner, the time required to reach a specified temperature threshold value is used as a measure of the pressure. Since time can be measured very precisely at low cost, this method has major advantages, such as extension of the measurement range, greater accuracy, and low power consumption. The described operating method is used in model VSP-62 Pirani vacuum gauges manufactured by Thyracont Vacuum Instruments GmbH.
However, the described vacuum gauges only evaluate the time required to reach a specified temperature during heating. The cooling time is not evaluated, since measuring the resistance of the Pirani element requires applying a measuring voltage, which interferes with the cooling of the element.
Therefore, in light of the problems associated with existing approaches, there is a need for improved systems and methods for accurately measuring pressures in a vacuum over a wide range.